Nevermind the fact that they also see this "venture" of his as nothing more than another way to pass time for these extravagant nobles. They said so before, that in the past other nobles also tried to make their own shops out of boredom and for shit and giggles, only for those stores to go out of business once the naive nobles get a taste of the hardships of the real world outside of their mansions, so having this guy's store flourish (besides the other economical, personal and social facts that previous commenters mentioned) makes them angry because they see him as another capricious noble that, for one reason or the other, seems to be getting lucky and/or abusing his influence.